1. Field of the Invention
The present invention relates to an ASK modulator, ASK modulators being utilized in such applications as car-mounted units and road-side radio units.
2. Description of the Related Art
Increasingly ASK (Amplitude-Shift Keying) modulations are being used centrally for narrow-band communications or for extremely short-range communications, such as in ETC (Electronic Toll Collection), keyless entry or RFID (Radio Frequency IDentification) tag systems. The advantage of ASK modulation is that the configurations of modulation/demodulation circuits can be made simple.
ASK modulations are a type of digital modulation system, so their signal waves are square waves. When carrier waves are directly modulated with square waves, the band of the carrier widens and this does not satisfy standards for adjacent wave leakage power and occupied band width. In order to confine the adjacent wave leakage power and the occupied band width to within the standard values, it is necessary to band-limit digital data.
To limit the bands of the digital data in ASK modulations, there is a method (as referred to in Japanese Patent Application Laid-open (JP-A) No. 2000-307664) using an analog LPF (Low-Pass Filter) and there is a method (as referred to in JP-A No. H5-136828) using a digital filter.
FIG. 14 shows an example of a conventional ASK modulator. FIG. 15 shows waveforms of the individual portions of FIG. 14. FIG. 15 shows the waveforms when the sending bit rate for sending binary data is 1,024 Kbps, and the sending data, from a data sending unit 51 shown in FIG. 14, is indicated by “DATA”. When this binary sending data is to be sent, Manchester-encoding is performed such that the sending data is changed at the center of a sending bit from “1” to “0” when a sending bit is “1”, and such that the sending data is changed in the opposite direction from “0” to “1” when a sending bit is “0”. By performing this Manchester-encoding in a Manchester encoder 52, the pulse change always occurs at the center of the bits so that synchronization can be easily made at the receiving side. If this Manchester-encoding is performed, two symbols are sent for a single bit time period. If the single bit time period is Tbit, one symbol time Tsymb is expressed by Tsymb=Tbit/2. The waveform, which is prepared by applying the Manchester-encoding to the waveform shown by “DATA”, is shown as “MANCHESTER” in FIG. 15. If the carrier waves are subjected to ASK modulations with this Manchester-encoded waveforms, side lobes of high energy are widely distributed, as shown by the frequency spectrum of FIG. 16. If such ASK-modulated waves are sent as they are as electric waves, they cause serious radio interferences in the near-by bands. Therefore, carrier waves which are ASK-modulated by Manchester-encoded signals are subjected to band-limitation in an LPF 53 in FIG. 14. FIG. 17 shows the frequency spectrum of the ASK-modulated waves of the band-limited signal. The ratio between the power within a bandwidth of 4.4 MHz at the center frequency and the power within 4.4 MHz at the frequency spaced by 5 MHz from the center frequency is the adjacent wave leakage power ratio, and is used as an index for measuring the energy of the side lobes.
Regarding the spectrum of FIG. 16, the adjacent wave leakage power ratio is about 16 dB. Regarding the spectrum of FIG. 17, however, the adjacent wave leakage power ratio is 38 dB. It is found that the energy of the side lobes is sufficiently lower than that of the frequency spectrum of FIG. 16.
In the DSRC (Dedicated Short-Range Communication System) and the like, it is necessary to implement a complicated modulation method such as QPSK (Quadrature Phase Shift Keying) modulation as well as the ASK modulation, thus band-limitation using a digital filter becomes the standard method. In a circuit for limiting the band with a digital filter, the response of the band-limiting filter is calculated in advance according to an input digital data sequence and is stored in a ROM. Then, a response is read out on the basis of the input digital data and is subjected to D/A conversion, so that a band-limited signal is generated. A band-limited signal generating circuit according to a stored waveform reading method is disclosed in JP-A No. H5-136828. FIG. 4 is a block diagram of a conventional band-limiting signal circuit, and FIG. 5 presents waveform diagrams of the individual portions of FIG. 4. In this example, a transient response of the signal caused by band-limitation extends over five bits. Thus, input serial data is stored in units of five bits. Each bit of the data “11010” is individually stored in five data registers #1 to #5 in a register 40 shown in FIG. 4. The data bit shifts in the data register 40 at a timing of a waveform “g”, which is a data clock waveform, shown in FIG. 5. Waveforms “a” to “e” are element waveforms of the individual bit units. The waveform “a” indicates the signal waveform of the data to be stored in the data register #1. Likewise: the waveform “b” indicates that of the data register #2; the waveform “c” indicates that of the data register #3; the waveform “d” indicates that of the data register #4; and the waveform “e” indicates that of the data register #5. In a ROM 41, a waveform, which is the additions of waveforms “a” to “e” at the position of the register #3, is stored in advance. The waveform stored in the ROM 41 is outputted as a band-limited output waveform data at a timing of a ROM clock waveform “f”, where the time period thereof corresponds to one-eighth of the time period of the waveform “g”.
When such an ASK-modulated signal is demodulated, a demodulator shown in FIG. 6 is used. In FIG. 6, the ASK-modulated waves are converted by a logarithmic envelope detector (LOG ENVELOPE DETECTOR) 60 into waveforms proportional to the logarithmic values of the envelopes of the modulated waves. These logarithmic envelope signals are averaged by an LPF 61 to create a slice level, and the slice level and the logarithmic envelope signal are compared in a comparator 62, to thereby demodulate the sent Manchester-encoded signals. The optimum slice level has an intermediate value between a High level and a Low level of the logarithmic envelope signal.
When the sending bit is consecutive “1”s, as described above, the Manchester-encoded signal changes with a time period one half of that of the Tbit. When the sending bit is a repetition of “0101”, the Manchester-encoded signal changes with a time period of Tbit. Thus, in the Manchester-encoded signal, signal sections of changes at one half of Tbit and signal sections of changes at Tbit are mixed. Therefore the Manchester-encoded signal is subjected to band-limitation at the LPF 53 of FIG. 14 so that the signal sections of changes at one half of Tbit have a smaller amplitude than that of the signal sections of changes at Tbit. Moreover, the On/Off ratio of the ASK-modulated waves, logarithmically measured, i.e., in decibels, is large for the signal sections of changes at Tbit, but small for the signal sections of changes at one half of Tbit. FIG. 7A shows the logarithmic envelope of the modulated output by the conventional ASK modulator. In FIG. 7A, the Low level of the signal sections for changes at Tbit is far lower than the Low level of the signal sections for changes at one half of Tbit. As a result, it is difficult to maintain the optimum slice level on the receiving side. If the Low level portion of the logarithmic envelope continues and if this continuous Low level portion is lower than the discontinuous isolated Low level portion, as shown by the slice level waveform in FIG. 6, the slice level deviates from the optimum slice level and this causes erroneous demodulation.
Here, if the difference in the On/Off ratio of the ASK-modulated waves between the signal section for the change of Tbit and the signal section for the change of one half of Tbit is reduced, the deviation from the optimum slice level in the receiving side is reduced. For example, there is a method for increasing the cut-off frequency of the band-limiting LPF 53 in FIG. 14 thereby to increase the On/Off ratio of the ASK-modulated waves in the signal section of the change for one half of Tbit. FIG. 18 shows the logarithmic envelope signal of ASK-modulated waves when the cut-off frequency of the band-limiting LPF 53 is increased. FIG. 19 shows the frequency spectrum of this case. In FIG. 18, as compared with FIG. 7A, the difference in the Low level between the signal sections of the changes at Tbit and the signal sections of the changes at one half of Tbit substantially disappears. In FIG. 19, however, the adjacent wave leakage power ratio is 28 dB and this is worse by 10 dB than that shown in FIG. 17.
In addition to the increase of the cut-off frequency of the band-limiting LPF 53 in FIG. 14, when the depth of modulation of the carrier waves in mixer 54 is lowered, the On/Off ratio of the ASK-modulated waves at the output of the mixer 54 is reduced. In this case, the deterioration of the adjacent wave leakage power ratio, due to the nonlinearity of a power amplifier 55 which comes after the mixer 54, is suppressed. However, the adjacent wave leakage power ratio to be inputted to the mixer 54 deteriorates, and hence the adjacent wave leakage power ratio of the output of the power amplifier 55 is not improved much, despite sacrificing the reduction of the On/Off ratio of the ASK-modulated waves.
The present invention is to solve the aforementioned problems, and it is an object of the present invention is, without introducing the deterioration of an adjacent wave leakage power, to realize an ASK modulator in which the difference between the On/Off ratio of the low envelope frequency components and the On/Off ratio of the high envelope frequency components is remarkably reduced, in order to generate easily receivable signals.